EGU2020-20837
https://doi.org/10.5194/egusphere-egu2020-20837
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Antarctic sea ice decline delayed well into the 21st century in a high-resolution climate projection

Thomas Rackow1, Sergey Danilov1,2, Helge F. Goessling1, Hartmut H. Hellmer1, Dmitry V. Sein1, Tido Semmler1, and Thomas Jung1,3
Thomas Rackow et al.
  • 1Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), Climate Dynamics, Bremerhaven, Germany (thomas.rackow@awi.de)
  • 2Jacobs University Bremen, Bremen, Germany
  • 3Institute of Environmental Physics, University of Bremen, Bremen, Germany

Despite ongoing global warming and strong sea ice decline in the Arctic, the sea ice extent around the Antarctic continent has not declined during the satellite era since 1979. This is in stark contrast to existing climate models that tend to show a strong negative sea ice trend for the same period; hence the confidence in projected Antarctic sea-ice changes is considered to be low. In the years since 2016, there has been significantly lower Antarctic sea ice extent, which some consider a sign of imminent change; however, others have argued that sea ice extent is expected to regress to the weak decadal trend in the near future.

In this presentation, we show results from climate change projections with a new climate model that allows the simulation of mesoscale eddies in dynamically active ocean regions in a computationally efficient way. We find that the high-resolution configuration (HR) favours periods of stable Antarctic sea ice extent in September as observed over the satellite era. Sea ice is not projected to decline well into the 21st century in the HR simulations, which is similar to the delaying effect of, e.g., added glacial melt water in recent studies. The HR ocean configurations simulate an ocean heat transport that responds differently to global warming and is more efficient at moderating the anthropogenic warming of the Southern Ocean. As a consequence, decrease of Antarctic sea ice extent is significantly delayed, in contrast to what existing coarser-resolution climate models predict.

Other explanations why current models simulate a non-observed decline of Antarctic sea-ice have been put forward, including the choice of included sea ice physics and underestimated simulated trends in westerly winds. Our results provide an alternative mechanism that might be strong enough to explain the gap between modeled and observed trends alone.

How to cite: Rackow, T., Danilov, S., Goessling, H. F., Hellmer, H. H., Sein, D. V., Semmler, T., and Jung, T.: Antarctic sea ice decline delayed well into the 21st century in a high-resolution climate projection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20837, https://doi.org/10.5194/egusphere-egu2020-20837, 2020

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Presentation version 1 – uploaded on 05 May 2020
  • CC1: question about your simulation, Sonya Legg, 07 May 2020

    Hi Thomas, I enjoyed hearing about your work. Just a couple of questions:

    -Did you run a single high res simulation or an ensemble of simulations? If only one simulation, do you think there might be internal variability in your results that could influence the outcome? 

    -With the increased eddies in your high res simulation, did you employ any lee-wave drag parameterization to dissipate the eddy energy? 

    Thanks!

    Sonya Legg

     

    • AC1: Reply to CC1, Thomas Rackow, 07 May 2020

      Dear Sonya,

      we did not employ any lee-wave drag parameterization to dissipate eddy energy. One reason is that ocean-only simulations on the high-resolution (HR) mesh show that our sea surface height variability is still lower than in observations (AVISO), so any extra dissipation is avoided. Secondly, the FESOM1.4 ocean-sea ice model is an A-grid model. It works with no-slip boundary conditions on all vertical walls, which is also rather dissipative. We will probably need lee-wave drag parameterizations on even finer computational grids we intend to use in the future. On our present grids, horizontal viscosity is very dissipative.

      Regarding the first question: It is a single high-res simulation (locally eddy-resolving ocean and atmosphere at T127), but we performed a second mixed-resolution "ensemble member" where the atmospheric resolution is smaller (T63) but the ocean resolution is unchanged, with very similar result. That gives us confidence that the quite large differences between LR and HR are robust and not largely due to internal variability. Moreover, we are looking at differences between the end of the 21st century (2070-2099) and the beginning (1990-2019). Current climate models typically do not show high internal variability on these timescales, as far as I know.

      Best wishes to Princeton!
      Thomas

      • CC3: Reply to AC1, Sonya Legg, 14 May 2020

        Thanks for the clarification!

  • CC2: Sea ice decline in the HR, Ivy Frenger, 14 May 2020

    Hi Thomas,

    Very interesting, thank you for the clear presentation.
    The HR does show sea-ice decline towards the second half of the 21st century, even though it is weaker then in the LR; can you briefly say why?
    I presume this is in the publication in review - I'll check out your paper once it is out :-).

    Best, Ivy